Concentrated glycols are made of almost 100% glycol. For example, Dow® inhibited propylene and ethylene glycols are composed of at least 94% glycol. The remaining 6% (or less) is made up of important corrosion inhibitors and purified water. This mixture is called concentrated inhibited glycol. When you purchase 275 gallons of DOWFROST™ HD for freeze protection up to -1° F, we would recommend you buy a premixed solution of 40% glycol (and 60% deionized water). Your 275 gallons of fluid contains 110 gallons of concentrated glycol (or 40%) and 165 gallons of deionized water (the remaining 60%). The 110 gallons of Dow glycol are pure (94% glycol and 6% inhibitors). Many companies sell lower purity (less concentrated) glycol, meaning that instead of a composition weight of 94% glycol in the unmixed form, you are buying a fluid that contains a lot of additives. The resulting composition weight of the glycol is more like only 65-70% glycol. A less concentrated glycol is less effective because customers assume they are buying 40% of a concentrated glycol mixed with deionized water, but when you take into account the diluted glycol composition, their mixture actually ends up to be only 28% glycol and 72% water. The lower price makes sense because you are buying 12% less glycol. Unfortunately, these companies do little to make it known that they are selling less concentrated product, hoping that this will be overlooked by a more attractive sticker price. The danger in this bait-and-switch is that your system will not be protected to the lowest anticipated temperature. 28% glycol only protects to about 15° F.